Here...let me save you some time...
The green circle is the correct result...your "eight inches per mile squared" is in red - both axes are in miles.
Like I said, it's the WRONG equation. Kinda-sorta-maybe-vaguely-right for VERY short distances...but not definitive.
The error doesn't look much on this graph - but bear in mind - the vertical scale is 500 *miles* to each grid square - and we're arguing about 10's of feet.
It's typical of an FE'er to CAREFULLY trim the graph so that his horrible error is just off the edges - and to hope that the rest of us are too stupid not to know the correct answer. Honestly...you must have peeked another 1000 miles off the the side...seen your problem...hoped none of us would call you on it.
Well, guess what Boots? We're not as dumb as you think we are!